The present invention relates generally to semiconductor fabrication and, more particularly, to a chemical mechanical planarization (CMP) apparatus and a method for performing a CMP process.
In the fabrication of semiconductor devices, planarization operations are often performed on a semiconductor wafer (xe2x80x9cwaferxe2x80x9d) to provide polishing, buffing, and cleaning effects. Typically, the wafer includes integrated circuit devices in the form of multi-level structures defined on a silicon substrate. At a substrate level, transistor devices with diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define a desired integrated circuit device. Patterned conductive layers are insulated from other conductive layers by a dielectric material. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to increased variations in a surface topography of the wafer. In other applications, metallization line patterns are formed into the dielectric material, and then metal planarization operations are performed to remove excess metallization.
The CMP process is one method for performing wafer planarization. In general, the CMP process involves holding and contacting a rotating wafer against a moving polishing pad under a controlled pressure. CMP systems typically configure the polishing pad on a rotary table or a linear belt. Additionally, a slurry is used to facilitate and enhance the CMP process. The slurry is introduced and distributed over a working surface of the polishing pad. Distribution of the slurry is generally accomplished by a combination of polishing pad movement, wafer movement, and pressure applied between the wafer and the working surface of the polishing pad.
FIG. 1A is an illustration showing a top view of a conventional rotary-type CMP system 101 implementing a polishing pad 103. The polishing pad 103 rotates in a direction 104. A wafer holder 105 attached to a spindle 107 is configured to rotate in a direction 108 above the polishing pad 103. A slurry manifold 109 is disposed above the polishing pad 103.
FIG. 1B is an illustration showing a side view of the conventional rotary-type CMP system 101. The polishing pad 103 is disposed on top of a rotary table 117. The rotary table 117 is supported by a spindle 119 capable of rotating in the direction 104. A wafer 113 is supported above the polishing pad 103 by the wafer holder 105. The wafer holder 105 is supported by the spindle 107, which rotates in the direction 108. During operation, a force 121 is applied to the spindle to cause the wafer 113 to contact the polishing pad 103. Also during operation, a slurry 115 is dispensed onto the polishing pad 103 from the slurry manifold 109. As the polishing pad 103 rotates in the direction 104, the slurry 115 is transported to the wafer 113.
FIG. 1C is an illustration showing a top view of the conventional rotary-type CMP system 101 in operation. During operation, the polishing pad 103 rotates in the direction 104 while the wafer holder 105 rotates the wafer 113 (see FIG. 1B) in the direction 108. Slurry 115 dispensed from the slurry manifold 109 onto the polishing pad 103 is transported to the wafer 113. Not all of the slurry 115 dispensed onto the polishing pad 103 is capable of traversing beneath the wafer 113. Thus, a slurry buildup 127 occurs at a front edge of the wafer 113. Due to the rotation of wafer 113, the slurry buildup 127 tends to wrap around the wafer 113 and becomes excess slurry 129. As the polishing pad 103 rotates, the excess slurry 129 moves toward and over an outer edge of the polishing pad 103 under the influence of centrifugal force. A similar situation exists in linear-type CMP systems in which excess slurry is thrown from a moving belt pad rotating around a pair of rollers. In general, less than 20% of the slurry 115 that is dispensed traverses beneath the wafer 113. The slurry 115 contribution to a total consumable cost of the CMP process can range from 60% to 80%. Therefore, a need exists to improve the efficiency of slurry utilization in the CMP process.
In addition to inefficient slurry use, maintaining a uniform temperature distribution across the wafer 113 is also a challenge with the rotary-type CMP system 101. As the polishing pad 103 traverses beneath the wafer 113, the polishing pad 103 will be exposed to heat being generated from friction and chemical reactions. As the polishing pad 103 rotates, a lower angular velocity exists at a radius r1 as compared to a radius r2. Thus, a unit surface area of the polishing pad 103 traversing beneath the wafer 113 at the radius r1 will be exposed to more heat than a unit surface area of the polishing pad 103 traversing beneath the wafer 113 at the radius r2. Hence, a temperature variation will develop across the polishing pad 103 from radius r1 to radius r2 as the CMP process continues. A similar situation exists in linear-type CMP systems in which a temperature variation can develop across a linear belt pad. However, in the linear-type CMP system, the temperature variation across the linear belt pad is due to a circular surface area of the wafer 113 that is in contact with the linear belt pad. Basically, outer regions of the linear belt pad traverse below smaller segments of the wafer 113. Thus, outer regions of the linear belt pad are exposed to less heat than inner regions. Hence, a temperature variation will develop across the linear belt pad from an outer region to an inner region as the CMP process continues. Since the CMP process is partially dependent on temperature, having a temperature variation across the rotary-based polishing pad 103 or linear belt pad may adversely affect the CMP process results. Rotation of the wafer 113 and slurry movement can help reduce the temperature variation, but not in a totally effective manner. Therefore, a CMP system is needed in which a more uniform temperature distribution can be maintained across a working surface such as the rotary-based polishing pad or the linear belt pad.
Many conventional CMP pads (i.e., rotary-based pads or linear belt pads) have pores dispersed therein. As a conventional CMP pad is used, inner planes of the conventional CMP pad become exposed, thus exposing the pores. In general, the pores in conventional CMP pads have a mean diameter of about 40 microns xc2x125 microns (1 micron=1E-6 meter). Many surface feature sizes on a wafer vary from about 0.3 micron to about 20 microns. Hence, the larger pore diameters contained within the conventional CMP pad are not satisfactory to provide ideal planarization. Further, as the pores are not evenly distributed throughout the conventional CMP pad, the surface area contact between the wafer and pad can change as a function of wear, causing uncontrolled variability to be introduced into the CMP process. Additionally, the conventional CMP pad has a root mean square (RMS) surface roughness of about 6 microns, which contributes to non-optimal planarization and surface roughness on the wafer. The RMS surface roughness of the conventional CMP pad also introduces difficulty in obtaining a desired wafer surface planarity as low as 0.01 micron. Thus, there is a need for a CMP pad that does not have large and/or uncontrolled surface properties that limit wafer planarization performance.
In addition to CMP pad surface characteristics, abrasives contained within the slurry (i.e., slurry abrasive) also have an effect on the CMP process. A solgel colloidal abrasive is a common type of slurry abrasive defined by discrete abrasive particles. The solgel colloidal abrasive particles can vary in diameter from 0.04 micron xc2x10.02 micron to 0.2 micron xc2x10.1 micron. A fumed aggregate abrasive is another common type of slurry abrasive defined by a string of linked abrasive particles having a typical length of about 0.25 micron xc2x10.1 micron. Some CMP processes may require that a wafer surface planarity of about 0.02 micron xc2x10.01 micron be obtained. In these instances, common slurry abrasive sizes such as those identified above can yield non-optimal planarization results. Thus, there is a need for a CMP process that can implement smaller abrasive particle sizes to more easily achieve a desired wafer surface planarity.
In view of the foregoing, there is a need for an apparatus and a method that can be implemented in a CMP process to improve the efficiency of slurry utilization, maintain a more uniform temperature distribution across the working surface to which the wafer is exposed, reduce large surface discrepancies on the working surface to which the wafer is exposed, and implement reduced abrasive particles sizes to achieve the desired wafer surface planarity.
Broadly speaking, the present invention fills these needs by providing a chemical mechanical planarization (CMP) apparatus including a bath in which the CMP operation is conducted. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several embodiments of the present invention are described below.
In accordance with one aspect of the invention, a CMP apparatus is provided. The CMP apparatus includes a bath of an aqueous solution. A first holder, which is configured to support a wafer, is disposed within the bath. A first spindle is configured to rotate the first holder. A second holder, which is rotated by a second spindle, is disposed above the first holder. The second holder supports a planarization media, which is disposed within the bath. The planarization media is oriented to face the surface of the first holder on which the wafer is to be supported. In one embodiment, the planarization media is a pad containing polyurethane. In another embodiment, the planarization media includes a substrate and an abrasive film overlying the substrate.
In one embodiment, the CMP apparatus further includes a system for recirculating and reconditioning the aqueous solution. In one embodiment, the system for recirculating and reconditioning the aqueous solution includes a chemical component analyzer, an auto-titration device, an auto-filtration device, a heat exchanger, and a pump. In one embodiment, the CMP apparatus further includes a device for monitoring a condition of a wafer to be supported by the first holder. The device can measure a wafer surface characteristic parameter such as, for example, film thickness, optical reflection, or an eddy current.
In accordance with another aspect of the invention, a method for performing a chemical mechanical planarization (CMP) process is provided. In this method, a wafer is immersed in a bath of an aqueous solution. A planarization media, which is oriented parallel to a plane of the surface of the wafer, is brought in compliance with the wafer. A portion of the wafer in compliance with the planarization media is then abraded. In one embodiment, the abrading is effected by rotating the planarization media in compliance with the wafer while holding the wafer in a fixed position. In another embodiment, the abrading is effected by rotating the planarization media in compliance with the wafer while rotating the wafer in an opposite direction relative to a planarization media.
In one embodiment, the method further includes circulating the aqueous solution. In one embodiment, the method further includes monitoring a concentration of the aqueous solution. In one embodiment, the method further includes reconditioning the aqueous solution by adjusting a concentration of the aqueous solution. In one embodiment, the method further includes monitoring a condition of the wafer in compliance with the planarization media.
The advantages of the present invention are numerous. Most notably, the CMP apparatus and the method of the present invention enable superior uniform planarization results to be achieved. The CMP apparatus provides an isothermal environment that significantly reduces temperature variations across the wafer surface and significantly reduces the shear forces exerted onto the wafer surface during the CMP process.
When an engineered planarization media, i.e., a planarization media having an abrasive film, is used, the improved surface property control of the planarization media enables a superior wafer surface planarity to be achieved. The CMP apparatus in conjunction with the engineered planarization media enables damage-free planarization processes involving relatively fragile wafer materials such as copper and low-k dielectric material. Planarization and film removal can now be performed more safely, i.e., without damaging the fragile wafer materials, through the use of the isothermal CMP apparatus and the engineered planarization media, which when combined provide more uniform and controlled friction distribution.
In addition, the CMP apparatus and the method of the present invention increase selectivity and thereby allow a CMP process to be self-stopping. In addition, flexibility in sizing of the planarization media can allow the CMP process to focus on a specific portion of the wafer. The CMP apparatus and the method of the present invention also offer economic advantages. For example, through recirculation and reconditioning, the aqueous solution in the CMP apparatus of the present invention is used more efficiently than conventional slurries are used in conventional CMP systems. Also, the engineered planarization media can be reclaimed to reduce the total consumable cost of the CMP process.
Other aspects and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the present invention.